1. Field of the Invention
The invention relates generally to global positioning system (GPS) receivers and more particularly to a GPS receiver apparatus having an improved time to first fix by using standard time and frequency information available from a radio signal.
2. Description of the Prior Art
Global positioning system (GPS) receivers are used in many applications where accurate time and geographical location are required. In several applications, it is important to minimize the time delay between when the GPS receiver is turned on and when it determines a GPS-based time and/or geographical location. This time delay, known as the time to first fix (TTFF) includes (i) time to tune the frequency of the GPS receiver to a carrier frequency of a GPS signal from a GPS satellite, (ii) time to align a phase of an internally generated pseudo-random noise (PRN) code to a PRN code in the GPS signal, (iii) time to receive data bits in the GPS signal to determine a GPS-based time, (iv) time to tune frequency and align phase to acquire a GPS signal from a second, a third, and a fourth GPS satellite, and (v) time to calculate a GPS-based geographical location. Fewer than four GPS satellites may be sufficient if the GPS receiver has other information such as altitude. The TTFF may include additional time delay if the GPS receiver does not already have an approximate time, its approximate geographical location, and ephemeris information for the locations in space of the GPS satellites. Existing GPS receivers have TTFFs in the range of a few tens of seconds to a few minutes.
The GPS signal is modulated with data bits at a fifty bits per second (BPS) rate (twenty milliseconds per bit) that are modulated by a coarse/acquisition (C/A) PRN code sequence at a 1.023 megahertz rate (one microsecond per chip) that is 1023 chips long (one millisecond). Each of the GPS satellites has a distinct PRN code that enables the GPS receiver to distinguish the GPS signal of one GPS satellite from the GPS signal of another GPS satellite. The data bits are organized into sub-frames that are six seconds in length. Each sub-frame includes a hand over word (HOW) that includes information for the GPS time of emission for a data bit. In existing GPS receivers, the time to receive the data bits to determine the GPS-based time includes up to six seconds to receive the GPS time in the HOW. Under some conditions the GPS receiver must receive more than one HOW, thereby adding more than one six second time increment to the TTFF.
The time to tune the frequency of the GPS receiver to the carrier frequency of the GPS signal depends upon the accuracy of an internal reference frequency. Some existing GPS receivers have eliminated or minimized this time by using a highly accurate internal reference oscillator such as an oven stabilized crystal oscillator or an atomic clock. However, such oscillators typically have a high power consumption and are expensive. Instead, most existing GPS receivers tune to the carrier frequency using a frequency search where first one frequency and then another is tried until a correct carrier frequency is found. Unfortunately, the time to do the frequency search increases the TTFF.
There is a need for a GPS receiver apparatus that determines GPS time without waiting for the HOW in the GPS signal and/or tunes to the GPS signal without using a highly accurate oscillator or frequency searching in order to have a fast time to first fix (TTFF).